Researchers at Stanford University have discovered that human embryonic stem cell therapies may suffer from a high probability of immune system rejection similar to that found in organ transplantation. The study, which one researcher called a “reality check,” could mean there will be significant delays in perfecting embryonic stem cell therapies.

Researchers found that an immune response resulted in mice injected with human embryonic stem cells (hESCs). Because of the immune response, all the transplanted cells were dead within a week, Scientific American reports.

The new findings, in addition to previous work, suggest hESCs injected into a human patient would also provoke an immune response. The U.S. Food and Drug Administration has not approved the injection of hESCs into human patients because the raw cells have the potential to become cancerous.

hESCs reportedly have the potential to mature into several different types of tissue, but they are harvested from human embryos that have been conceived and then destroyed to provide the stem cells.

Stanford radiologist Joseph Wu said the study shows that such cells do not “slip under the radar” of the immune system. Some scientists had hypothesized that embryonic cells avoid immune system reactions because half of the naturally conceived human embryo’s genes are foreign to its mother.

In the recent Stanford study, researchers used a noninvasive molecular imaging technique to monitor the cells injected into mice in order to determine exactly when the injected hESCs died off.

When hESCs were injected into mice with non-functioning immune systems, the cells reportedly thrived and multiplied. When injected into mice with functioning immune systems, the cells began to die within a week and were completely gone within ten days. When more hESCs were injected into the same mice, the cells then died within two to four days.

Noting the immune reaction was similar to that found during organ transplantation, the researchers repeated the experiment in animals that received two common anti-rejection medications: tacrolimus and sirolimus.

The hESCs lasted 28 days in such animals, but it is not known if that is enough time to have a therapeutic effect.

“Our only finding here is that this combo of drugs can have a medicating effect on these cells,” Wu said.

According to Scientific American, Wu said the result is "not a disappointment, it's more of a reality check… I think there's some promise [to hESCs], but you don't want to be foolish and say these cells are going to cure things in the next five years.”